This invention relates to pulse tube refrigeration systems and more particularly to the construction of a flow smoother for pressurized gas as it enters either the warm or cold end of a pulse tube expander.
One can think of a portion of the gas in a pulse tube as forming a piston that replaces the solid piston in a GM or Stirling expander. This concept is feasible and has been reduced to utilitarian practice if the gas enters the pulse tube alternately at both ends with a uniform flow pattern and very small cells of turbulence. Then flow is laminar and mixing of the gas in the pulse tube is minimized. By xe2x80x9cuniform flowxe2x80x9d is meant that the velocity of the gas at the tube entrance is substantially uniform over a face area of several square millimeters.
However, gas entering the first and second stages of a two-stage pulse tube system often flows through entrance passages with 90xc2x0 bends, for example, that tend to produce uneven flow distributions. The flow must be redistributed to a uniform flow for effective pulse tube operation. Prior art flow smoothers include perforated plates, holes drilled in a plate(s) as in U.S. Pat. No. 6,082,117, and sintered spheres, used by the inventor here in early development work on pulse tube expanders. These constructions have been used both as flow smoothers and heat exchangers.
What is needed is a flow smoother upstream of a pulse tube expander that turns turbulent flow into laminar flow so that mixing of gas in a pulse tube is minimized.
The warm and cold ends of a pulse tube expander should have a structure that promotes uniform flow of the gas entering the tube, with a minimum amount of turbulence. Layers of fine mesh wire screen, for example 200 mesh (200xc3x97200 wires/inch), would be desirable at an entrance to the expander, but individual screens are too flexible to be used separately. Therefore, coarser screens, perforated plates, or sintered spheres have been used in earlier development.
The present invention improves upon earlier methods of smoothing the flow by diffusion bonding several layers of fine mesh screen together to form a rigid disc or plate that is self supporting. The design can be refined by having several layers ranging from fine screens on the surface facing the pulse tube to coarser screens that provide structural support without significantly adding to the pressure drop across the screen assembly.
The flow smoother is typically used in series with a heat exchanger at the warm end that rejects heat to ambient from the gas of the pulse tube and a heat exchanger at the cold end that receives heat from the load being cooled. The screen assembly flow smoother can be physically extended in the flow direction to serve this heat exchanger function. Preferably a flow smoother is used in conjunction with a slotted heat exchanger of separate construction that has less void volume and less pressure drop than a screen heat exchanger for a given temperature difference produced by the exchanger.
The desirable characteristics of good heat transfer and uniform flow distribution in a flow smoother and in a heat exchanger come at the cost of pressure drop and void volume. Void volume is that portion of gas in the refrigerating system that is repetitively pressurized and depressurized without any advantage or purpose in operation of the refrigeration system, analagous to the clearance volume in an internal combustion engine cylinder.
Wire screens provide good heat transfer between the gas flow and the screens but the temperature difference between the center of the screens and the edge is large when compared to heat exchangers with a radial slot pattern. The difference between the two geometries increases as the diameter of the heat exchanger increases, favoring the slotted heat exchanger.
For a given overall temperature difference, the slotted heat exchanger has the lowest pressure drop and void volume relative to wire screens, perforated plates, or sintered spheres. Although the flow distribution exiting a slotted heat exchanger is not as good as with the other heat exchanger constructions, when an integral wire screen flow smoother of the present invention is placed between the heat exchanger and the pulse tube, overall properties are optimized.
Thus in accordance with the invention, a flow smoother, constructed of screens that are bonded together to form a self-supporting integral structure, is positioned at each end of a pulse tube expander. At the cold end of the expander, a heat exchanger may be combined with the smoother, positioned down stream of the flow smoother, or may immediately follow the smoother as an independent structure. The flow smoother screens are graded, with the finest mesh screens located on the pulse tube side of the smoother and the coarsest screens on the heat exchanger side.
A slotted heat exchanger that follows the flow smoother provides good heat transfer, low-pressure drop, and minimal void volume. That is, the flow smoother is positioned between the pulse tube and the heat exchanger. The slotted exchanger also helps to distribute the gas flow so that the flow smoother does not have to be as thick in the flow direction to provide uniform flow as compared to performance with other heat exchanger geometries.
Slotted heat exchangers have been described previously in a paper by G. Thummes et al, titled xe2x80x9cEffect of Pressure Wave Form on Pulse Tube Refrigerator Performancexe2x80x9d in Cryocoolers 8, pages 383-393, Plenum Press, New York 1995.
Accordingly, an object of the present invention is to provide an improved flow smoother for a pulse tube expander by maintaining uniform flow entering the pulse tube and avoiding large scale mixing and circulation of gas.
Another object of the invention is to provide an improved flow smoother having fine mesh screens at the interface between the flow smoother and pulse tube expander to minimize the scale of local turbulence.
A further object of the invention is to provide an improved self-supporting fine screen flow smoother with low pressure drop and little void space for use with a pulse tube expander.
Still other objects and advantages of the invention will be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.